A frame structure for a backrest, particularly a backrest of a vehicle seat, is expected to be lightweight to reduce the weight of a whole vehicle. Also, it is expected to have a high enough stiffness to properly protect passengers at the moment of a lateral or rear-end vehicle collision, in addition to being required to have an adjusting mechanism to adjust the backrest angle to rotate around a lower supporting point.
If such a frame structure is chiefly made of metal, the frame structure will have a comparatively complicated shape as disclosed in JP 2010-94436-A and, therefore, members of the frame will need to be bonded with many processes such as welding to meet the above-described demands. Besides, the structure chiefly made of metal cannot be greatly reduced in weight.
On the other hand, FRP (Fiber-Reinforced Plastic) is a lightweight and high stiffness material known as an alternative to metal, and some prototypes of frame structures for a backrest have been made of FRPs as disclosed in JP 2005-194-A, JP 2010-500198-A and JP 2010-220748-A. JP '194 discloses a structure in which convex upper reinforcing portions are formed integrally with both side ends of a plate-shaped framework. In such a structure, because the framework is made with a plate member extending to a broad area, the plate thickness of the framework has to be increased to improve the stiffness of the backrest. Therefore, the overall weight savings that can be achieved are limited. JP '198 discloses a structure in which frame parts (corresponding to Side Frames positioned in corresponding places of our structures) disposed on both left and right sides of the backrest structure have a significantly enhanced stiffness. However, because the stiffness of a frame part between the side frames (corresponding to Transverse Frame positioned in a corresponding place of our structures) has not been specifically improved, the stiffness of the backrest structure might be insufficient as a whole especially on the left and right sides. Such a structure disclosed in JP '198 in which many ribs are provided to improve the left and right frame parts in stiffness do not have an excellent formability. In addition, there is a great amount of material usage that causes unfavorable production costs. JP '748 discloses a frame manufactured through a burdensome process in which a mandrel on which a three-dimensional braiding has been formed is picked out, squeezed and impregnated with resin at the RTM process. Concerning conventional structures disclosed in JP '194, JP '198 and JP '748, there is room for improvement in efficiently producing a lightweight and high stiffness frame structure at cheaper prices with ease.
It could therefore be helpful to provide a frame structure and its manufacturing method at a cheaper price with ease, particularly by structurally improving a transverse frame as a portion extending between side frames on both sides, additionally, by allocating reinforcing fibers to improve the stiffness as efficiently achieving weight savings as a whole.